Along with recent trends to more compact and thinner portable devices such as mobile phones, reproduction devices installed in mobile phones are getting thinner. However, there is an increasing demand for high volume sound reproduction as the usage of mobile phones diversifies into usages such as for listening to music and watching videos. Vibrations of reproduction devices are, however, transferred to cases of mobile phones due to reproduction of speech at high volume with small reproduction devices, with this leading to breakup (crackling noise) in the reproduced sound. The sound quality of reproduced sound deteriorates when breakup occurs often, and for example spoken voices become hard to catch with mobile phones.
There is accordingly a proposal for a crackling noise prevention method to prevent crackling noise generated within a vehicle due to low frequency sound signals contained in audio signals. In such a crackling noise prevention method, a low frequency crackling test signal is broadcast into a vehicle by reproduction with a speaker, and the crackling noise that is generated within the vehicle is collected by a microphone provided inside the vehicle. Then a fluctuation signal of amplitude fluctuations in the crackling noise signal generated by vibration in resonance with the low frequency signal is detected by the microphone collected signal, and the characteristics of low frequencies of a frequency characteristic adjuster (equalizer) input with the audio signal are controlled such that the fluctuation amount of the fluctuation signal achieves a set value or lower.